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Female Prussian carp steal sperm from related species

The Prussian carp is an invasive species in Europe, but unlike the common carp - the common species of carp, the females breed in virgin breeding. But it turns out that even in order to stimulate virgin reproduction, some kind of carp seed is needed.

The Prussian carp. Virgin reproduction with a twist. Courtesy of researchers from the University of Innsbruck, Austria
The Prussian carp. Virgin reproduction with a twist. Courtesy of researchers from the University of Innsbruck, Austria

The Prussian carp (Carassius gibelio), which originates from Asia, is an invasive species in Europe. It is a close relative of the goldfish, and competes with the endangered native common hook (a species of carp) for the same habitat. But, while goldfish and hookfish usually reproduce sexually, the Prussian carp has a major evolutionary advantage: the females can spare the time-consuming search for a mate.

Instead, the female Prussian carp use the sperm of common carp males, or of other carp species. To do this, they mix with a flock of roaches, where the eggs they laid are fertilized by the males.

The captured sperm stimulates the egg cell of the Prussian carp to divide. The male's genetic material is then broken down in the egg cell without being used. This is called sperm-dependent virgin reproduction. All the offspring created in this way are female clones of the female Prussian carp. Most of the Prussian carp populations are therefore only females, males appear only rarely.

"Unisexual reproduction (of females only) enables rapid settlement in new habitats and gives invasive species a great advantage over their original competitors," explains Donia Lemch from the Department of Lake Research at the University of Innsbruck in Austria. Her research focuses on studying the mechanisms of same-sex reproduction in organisms that live in water.

In a study initiated by Lemch, she and an international team of researchers succeeded in deciphering the entire genome of the Prussian carp. As a result, the mechanism behind asexual reproduction can now be better understood.

The complete hereditary information of an organism, the genome, is divided into different sets of chromosomes. Animals that reproduce sexually usually have a duplicate set of chromosomes. For reproduction, the chromosomes of females and males divide into primary cells (meiosis) and only one single set of chromosomes is transferred at a time. The fusion of a single-serial egg and a single-serial sperm creates a double-serial organism again.

But, glitches during meiosis or hybridization of related species repeatedly result in organisms that have more than two sets of chromosomes. Such higher vertebrates are unable to exist, but fish, amphibians and reptiles do. Even new species can develop like this - like the Prussian carp.

The Prussian carp has six sets of chromosomes. Four of them came together by hybridizing unrelated fish species—the other two were added by hybridizing with closely related fish.

"Probably at some point in these hybrids, there were problems with the formation of the gametes. This can be one of the factors of same-sex reproduction," Lemch explains. "In species that reproduce by virgin reproduction, meiosis fails and there is no longer a need for the fusion of the gametes."

The study made it possible to dissect the genome of the Prussian carp into individual chromosome series. This is the first time that all the genetic information of an animal with six sets of chromosomes has been described, and all six have been studied. The genome of the Prussian carp includes 150 chromosomes, more than three times the human genome.

for the scientific article

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